Conventionally, the mono-component development method which uses only a toner as the developer and the dual component development method which uses a toner and a carrier have been known as development methods in image forming apparatuses using the electro-photography system.
The mono-component development method is advantageous in terms of simplicity, compactness, and low cost of the apparatus. However, toner deterioration can be accelerated by the strong stress on the regulating section that charges the toner, and the charge-accepting ability of the toner thereby can be to decrease. Furthermore, because the surfaces of the toner regulating member and of the toner-supporting member can be contaminated with the toner and external additives, the ability to charge toner can be reduced and the life-span of the developing device is accordingly shortened.
In the dual component development method, because toner is charged by triboelectric charging caused by being mixed with carrier, the stress is smaller and this is advantageous against toner deterioration. Furthermore, because the surface area of carrier is large, the carrier is relatively resistant to contamination of toner and external additives, and this is advantageous in extending the life-span.
However, in the dual-component development method, when the latent electrostatic image on the image supporting member is being developed, the surface of the image supporting member is rubbed with the magnetic brush of the developer, and thus a trace of a magnetic brush impressions can be generated. Furthermore, the carrier tends to attach to the image supporting member and thus causing a problem of image defects.
A hybrid developing method has been disclosed (Refer to Unexamined Japanese Patent Application Publication No. H 05-150636) to maintain a long life-span of the two component development, where development is performed with only toner which is supplied to the toner supporting member from a dual-component developer supported on the developer supporting member.
However, the hybrid development method is problematic in that the post-development residual toner, on the toner supporting member, that has not been used in development appears on the image as development hysteresis (ghost image) in the subsequent development process. This is caused by insufficient toner collecting capability of the developer supporting member stemming from applying a bias to the developer supporting member so as to put a priority on supplying the toner to the toner supporting member.
In recent years, in order to solve this problem i, a method has been proposed in which the toner-collecting developer supporting member to which a voltage for collecting the post-development residual toner is applied is added in the hybrid development method (Unexamined Japanese Patent Application Publication No. H 10-319708). In the method described in Unexamined Japanese Patent Application Publication No. H10-319708, the problem of development hysteresis does not occur in the beginning because the post-development residual toner on the toner supporting member is surely collected by the toner-collecting developer supporting member.
However, in this method, because a voltage is continuously applied to the toner-collecting developer supporting member to attract toner in the direction thereto, the toner collected from the toner supporting member or the toner separated from the carrier in the developer are transferred to the surface of the toner-collecting developer supporting member, and thereby causing uneven distribution.
In this state where the toner is unevenly (much) distributed on the surface of the toner-collecting developer supporting member, when the developer is removed from the toner-collecting developer supporting member, the unevenly toner is not removed and remains on the surface of the toner-collecting developer supporting member. As a result, a long time use causes toner to accumulate on the toner-collecting developer supporting member.
Further, when stored for a long time, the problem arises that the accumulated toner films the surface of the toner-collecting developer supporting member.
As another solution, in order to solve the problem of the ghost-image generation, a method has been proposed in which multiple toner-supplying developer supporting members are used in succession to supply a large amount of toner to the toner supporting member (Unexamined Japanese Patent Application Publication No. 2007-34098).
However, also in this method, collection of the post-development residual toner was insufficient, and the method cannot be considered appropriate as a measure for dealing with the problem of ghost images.
As described above, technological improvements have been carried out in order to deal with ghost-image generation in the hybrid development method, but no technology has been proposed to satisfactorily meet the requirement.
An object of the present invention is to solve the above problems and to provide a developing device and an image forming apparatus in which the hybrid development method with a toner-collecting developer supporting member is employed, the toner collection capability is maintained for a long time, and a high quality image without development hysteresis (ghost) is obtained for a long time.